1. Field of the Invention
This invention relates to the formation of a paper web from an aqueous slurry of wood pulp fibers, commonly called stock. More particularly, this invention relates to a method and apparatus for the high-speed formation of paper at the initial stage of such formation by projecting a stock stream against (between) a traveling forming wire(s) at a location over the porous surface of a forming shoe. Still more particularly, this invention relates to such formation of a paper web utilizing a forming shoe wherein the porous surface comprises grooves in the face of the forming shoe supporting the forming wire, which grooves extend substantially in the direction of forming wire travel, but at a small angle thereto. In another preferred embodiment, the porous surface comprises a plurality of openings.
2. Description of the Prior Art
In the making of paper from an aqueous slurry of wood pulp fibers, whether the initial formation is done over a single forming wire, such as in a Fourdrinier forming section, or in a two forming wire machine, such as a so-called gap former, wherein a pair of looped, opposed forming wires are directed into a converging, co-running path of travel over a stock stream which is projected by a headbox between the forming wires, the water in the stock is drained through the forming wire(s) to begin the formation of the paper web by leaving the wood pulp fibers randomly distributed on the forming wire, or between the co-running forming wires.
Depending on the type of paper or paper board to be manufactured, different types of stock are used. The rate at which water can be removed from different stocks to produce a quality paper product is a function of many factors, such as, for example, the paper product, the desired caliper of the paper product to be made, the design speed of the papermaking machine, and the desired levels of fines, fibers and fillers within the final paper product.
The use of forming shoes to guide one or two forming wires in the forming section of a papermaking machine is known in the art. Also known is the use of a so-called forming roll, which is sometimes constructed of a foraminous cover for receiving water passing through the forming wire and into the forming roll from the stock carried on the outer surface of the forming wire.
It is further known to use a forming shoe having grooves in the surface thereof, which grooves begin downstream of the leading edge of the forming shoe and extending at a small angle to the machine direction (i.e., the direction of travel of the paper web through the papermaking machine).
Within the forming section of a papermaking machine, there is known various types of apparatus, such as foil blades, vacuum boxes, turning rolls, suction rolls, and open surface rolls which have been used in various configurations and sequences in order to seek optimization of the rate, time and location of removing water in the formation of the nascent paper web. Papermaking is still part art and part science in that simply removing water as fast as possible does not produce a paper product of the highest quality. In other words, the production of a high quality paper product at high speeds, such as, for example, about 6,000 ft/min. (2,000 m/min) is a function of the rate of water removal, the manner in which water is removed, the duration of water removal, and the location at which water is removed from the stock on the forming wire, or between the forming wires.
In the past, when papermaking machine speeds were lower, such as, for example, 3,000-4,000 ft/min. (914-1219 m/min), the relative application of the aforementioned factors might be different to produce the desired quality in the paper product. Further, as with most processes, when it is desired to maintain, or improve, quality of a product while producing the product at faster rates, unanticipated problems are often encountered which result in either the rate of production having to be lowered in order to maintain or attain the desired quality, or the desired quality having to be sacrificed in order to attain a higher rate of production.
Prior blade elements, or foils, for forming shoes, whether the forming shoe is curved or flat in surface configuration, sometimes contain a plurality of slots formed between a plurality of blade elements extending longitudinally along the length of the blade elements. The slots in turn define leading edges on the blade elements which are arrayed in the cross-machine direction perpendicular to the direction of forming wire travel. Such an arrangement works well. The stock stream is projected against a forming wire over the leading edge of the forming shoe/foil such that a portion of the stock stream passes through the forming wire and beneath the shoe/foil. Each foil, blade element, or forming shoe is either open at the bottom to atmospheric pressure, or they are connected to a source of sub-atmospheric pressure to enhance the dewatering process by urging the water into the slots between adjacent foils or blade elements defining the faces, or top surfaces, of the foil or forming shoe.
However, as papermaking machine speeds increase to more economically manufacture the paper product, new phenomena regarding the runnability of the papermaking apparatus as well as the appearance and internal structure of the paper product produced begin to appear. Most of these changes are not desirable.
These phenomena can take various forms, such as undesirable distribution of fines and fillers in the surface or interior of the paper product, and the first pass retention or retention of fine material would decrease. These variations and imperfections are deleterious to the paper product and affect its saleability.
The above-mentioned imperfections, deficiencies and factors affecting the production and quality of a paper product caused by a forming shoe or foil section in the forming section of the papermaking machine have been obviated or mitigated by this invention.
In this invention, a forming shoe is used which has a porous surface. In a preferred embodiment, the porous surface can take the form of a plurality of parallel grooves formed in a portion of its face surface. In another preferred embodiment, the porous surface can take the form of a plurality of small openings, such as drilled holes, slots, honeycomb, or the like.
The forming shoe has a curved, leading nose surface and the grooves, in a preferred embodiment, are initially formed in the downstream portion of the nose with their beginning (i.e., the bottom surface of the groove) smoothly contiguous therewith. The grooves extend downstream at a small angle to the machine direction, which is the direction of forming wire travel. The depth of the grooves also gradually increases from the point of their initial intersection with the nose surface on the forming shoe.
In a preferred embodiment, each groove does not extend through the forming shoe to be exposed to atmospheric pressure beneath the forming shoe. Further, in a preferred embodiment, each groove extends at its small angle to the machine direction for a distance such that the beginning of the groove, in the machine direction, overlaps the end of at least one adjacent groove such that a given point of the forming wire traveling in the machine direction passes over a portion of at least two grooves in its path of travel over the forming shoe.
Further in a preferred embodiment, the radius of curvature of the porous forming shoe is a compound radius, such as, for example, on a forming shoe having a face surface extending about 18 inches in the machine direction, a radius of up to about 60 inches, preferably about 30-40 inches, for the first four inches of length in the machine direction, and a radius of about 100-200 inches for the next ten to twelve inches downstream in the machine direction, and a radius of about 10 inches for the last two to four inches of face surface length. However, it is contemplated, and intended to be within the scope of this invention, that the compound radius could comprise two radii and two separate blades in the shoe, each blade being about seven inches long in the machine direction. There would be a small slot between the blades such as, for example, about one inch, or less. The radii would then be, for example, a 40 inch radius for the first four inches of face surface, and a radius of about 100-200 inches for the remainder of the face surface in a forming shoe having a total length of about 15 inches.
It is also contemplated that the radius of curvature change continuously, in the manner of a French curve, from the leading, or nose portion of the forming shoe, through the intermediate, or porous portion of the forming shoe, and through the trailing portion of the forming shoe, which may be porous or non-porous. This would be a continuously changing compound curve. The instant radius of curvature at any given location would be such that the rate of water removal at the point of stock stream impingement, and over the porous portion would be constant, or near constant, as desired.
Further, it is also contemplated that the curvature of the grooved forming shoe could comprise a simple curve for the nose portion with a substantially straight trailing surface, or a continuous curve. The straight railing surface configuration would probably only be used in a single forming wire application. The length of the straight surface would probably be no longer than about seven inches. For example, the radius of such a continuous curve for the face surface could range from about 25-60 inches for a face surface about eighteen inches in length. These are intended to be within the scope of the invention.
By not having the individual grooves extending substantially in the cross-machine direction, in conjunction with the radii described above, and with each location of the forming wire traveling in the machine direction, the stock carried on the outer surface of the forming wire passes relatively gently over a groove, in a dewatering action, since the groove co-extends in substantially the same direction for a relatively short period of time of forming wire travel, but which period of time is longer than the period of time at which the slot would pass under the stock if the slot was extending in the cross-machine direction. The machine direction nature of the grooves redirects the flow of the drained water less, which means less flow being forced back up into the web as the drained flow impacts the blade.
In the case of the face surface being porous by means of a plurality of openings, such as small holes, the small size of the individual openings, relative to the area of the face surface which does not contain small openings, provides the same benefit. As shown in FIG. 11, the slots are angled to avoid backflow, refluidization of the web, and a stripping of fine material.
Regardless of the contemplated configuration of the porous surface, the invention further embodies the concept of impinging the stock stream onto the curved face surface of the forming shoe over the porous surface and not over the leading edge of the forming shoe, as is done in the prior art.
In addition, the rapid pulsation in the stock on top of the forming wire in prior arrangements caused by the rapid alteration of the slots and the following land areas in foils, foil boxes or forming shoes is mitigated in this invention because a small area of stock, that is a small area of an aqueous slurry of wood pulp fibers, on the forming wire is exposed to the plurality of grooves or other means forming the porous surface for a somewhat longer period of time due to the ability of the porous surface to absorb the force of impingement of the stock stream by virtue of passing a portion of the water into the porous surface and thereby lessen the formation of any pulse. This pulse absorption takes the form of either the grooves extending at a small angle to the direction of machine travel such that the on-coming leading edge of the next successive blade element does not pass a particular line in the cross-machine direction at the same time, or the impinging stock being on the forming wire over openings in the porous surface.
This operation also functions to even out cross-machine paper web basis weight variations as well as mitigates pulsations in the stock passing over the face surface of the forming shoe. It helps to permit faster papermaking machine speeds while maintaining, or even improving, paper web formation.
Accordingly, it is a feature of this invention to provide a method and apparatus for improving the dewatering of stock in the forming section of a papermaking machine to form the nascent paper web in the early stage of paper formation when the headbox is discharging a stream of stock onto the forming wire over the porous forming shoe.
It is another feature of this invention to provide a method and apparatus for forming a paper web by use of a forming shoe having a porous surface.
It is another feature of this invention to provide a method and apparatus for forming a paper web by removing water from the stock by means of a forming shoe having a plurality of grooves extending at a small angle to the machine direction.
It is another feature of this invention to provide a method and apparatus for forming a forming shoe having a surface containing a plurality of small openings.
Yet another feature of this invention is to provide a method and apparatus for forming a paper web by use of a forming shoe having a porous surface which provides substantially constant water drainage in the downstream direction.